Page 464 - Mechanical Engineers' Handbook (Volume 4)

7 System Components 453

Oil Return
When the evaporator is used with reciprocating-compression equipment, it is necessary to
ensure adequate oil return from the evaporator. If oil will not return in the refrigerant flow,
it is necessary to provide an oil reservoir for the compression equipment and to remove oil
mechanically from the low side of the system on a regular basis. Evaporators used with
centrifugal compressors do not normally require oil return from the evaporator, since cen-
trifugal compressors pump very little oil into the system. However, even with centrifugal
equipment, low-temperature evaporators eventually may become contaminated with oil,
which must be reclaimed.

Two-Phase Refrigeration Distribution
As a general rule, the refrigerant is introduced into the evaporator by expanding it from a
high-pressure liquid through the expansion device. In the expansion process, a significant
portion of the refrigerant flashes off into vapor, producing a two-phase mixture of liquid and
vapor that must be introduced properly into the evaporator for satisfactory performance.
Improper distribution of this mixture can result in liquid carryover to the compressor and in
damage to the exchanger tubes from erosion or from vibration.

Vapor–Liquid Separation
To avoid compressor damage, the refrigerant leaving the evaporator must not contain any
liquid. The design should provide adequate separation space or include mist eliminators.
Liquid carryover is one of the most common sources of trouble with refrigeration systems.

Submergence Effect
In flooded evaporators, the evaporating pressure and temperature at the bottom of the ex-
changer surface is higher than at the top of the exchanger surface, owing to the liquid head.
This static head, or submergence effect, significantly affects the performance of refrigeration
evaporators operating at extremely low temperatures and low-suction pressures.
Beyond these basic refrigeration-design requirements, the chemical industry imposes
many special conditions. Exchangers frequently are applied to cool highly corrosive process
streams. Consequently, special materials for evaporator tubes and channels of particularly
heavy wall thickness are dictated. Corrosion allowances in the form of added material thick-
nesses in the evaporator may be necessary in chemical service.
High-pressure and high-temperature designs on the process side of refrigerant evapo-
rators are frequently encountered in chemical-plant service. Process-side construction may
have to be suitable for pressures seldom encountered in commercial service. Differences
between process inlet and outlet temperatures greater than 55 C are not uncommon. In such
cases, special consideration must be given to thermal stresses within the refrigerant evapo-
rator. U-tube construction or floating-tube-sheet construction may be necessary. Minor
process-side modifications may permit use of less expensive standard commercial fixed-tube-
sheet designs. However, coordination between the equipment supplier and chemical-plant
designer is necessary to tailor the evaporator to the intended duty. Relief devices and safety
precautions common to the refrigeration field normally meet chemical-plant needs, but should
be reviewed against individual plant standards. It must be the mutual responsibility of the
refrigeration equipment supplier and the chemical-plant designer to evaluate what special
features, if any, must be applied to modify commercial equipment for chemical-plant service.
Refrigeration evaporators are usually designed to meet the ASME Boiler and Pressure
Vessel Code, 21 which provides for a safe, reliable exchanger at economical cost. In refrig-
eration systems, these exchangers generally operate with relatively small temperature differ-
entials for which fixed-tube-sheet construction is preferred. Operating pressures in refrigerant
evaporators also decrease as operating temperatures are reduced. This relationship results in